The present disclosure relates to an offset cancel circuit which cancels an offset voltage contained in output signals of a bridge resistance type sensor configured by bridge-connecting a plurality of resistance elements.
Conventionally, a magnetic sensor is known in which a plurality of GMR elements (Giant MagnetoResistive effective elements) are bridge-connected (for example, JP-A-2007-212275). In a GMR element, in accordance with an external magnetic field, the magnetization direction of the free layer relatively rotates with respect to the fixed magnetization direction of the pinned layer, so that the electric resistance is changed. When attention is focused on the electric characteristics of a GMR element, a GMR element is one kind of resistance element. In the case where a plurality of GMR elements constitute a bridge circuit, therefore, an external magnetic field can be sensed by a change in potential balance of the two midpoint nodes of the bridge circuit.
However, GMR elements are dispersed in resistance. In a state where an external magnetic field is not applied, therefore, the potential difference between the two midpoint nodes of a bridge circuit becomes not zero, and an offset voltage is generated. When an external magnetic field is to be measured by the output of a magnetic sensor, consequently, the offset voltage must be cancelled.
FIG. 4 shows an example of a related offset cancel circuit 95. The related offset cancel circuit 95 includes current sources 96, 97 which are connected respectively to two midpoint nodes a, b of a magnetic sensor 90 configured by bridge-connecting four GMR elements 91, 92, 93, 94. The current sources 96, 97 cause preset constant currents I1, I2 to flow respectively through the two midpoint nodes a, b in the bridge circuit, thereby cancelling an offset voltage Voff appearing across output terminals A, B. Since the dispersion of the resistances of the GMR elements 91 to 94 can be previously measured, namely, the offset cancel circuit 95 adjusts the currents I1, I2 produced in the current sources 96, 97, based on the dispersion which is previously measured, and generates a voltage drop corresponding to the dispersion in the bridge circuit, to cancel the offset voltage Voff.
In the related offset cancel circuit 95, however, the current sources 96, 97 for causing the currents to flow through the magnetic sensor 90 are formed on a semiconductor chip by a CMOS process, and therefore the materials of the internal resistances for generating the constant currents in the current sources 96, 97 are different from those of the GMR elements 91 to 94 through which the constant currents flow. As a result, the resistances of the GMR elements 91 to 94 exhibit temperature characteristics which are different from those of the internal resistances of the current sources 96, 97. When the temperature is changed, therefore, the currents I1, I2 generated in the current sources 96, 97 have values which are deviated from values that are adequate to cancel the dispersion of the resistances of the GMR elements 91 to 94. Therefore, the related offset cancel circuit 95 has a problem in that it is difficult to, in an arbitrary temperature range, adequately generate a voltage drop corresponding to the resistance dispersion, in the bridge circuit, and the offset voltage Voff can be cancelled only at a specific temperature.
This problem is not limited to the magnetic sensor 90 which is configured by bridge-connecting the plurality of GMR elements 91 to 94, but this problem is also applied to bridge resistance type sensors in which a plurality of resistance elements are bridge-connected.